Digital oscilloscopes, and particularly digital storage oscilloscopes, and logic analyzers frequently are used to measure relative characteristics between a fixed reference signal and another signal which varies over time relative to the reference. Many of these variable signals are repetitive, in which case a variety of techniques can be used to determine relative characteristics.
One technique frequently used for reconstructing repetitive waveforms is referred to as random-repetitive sampling. In random repetitive sampling, the digitizing clock provides a reference signal, and the waveform is reconstructed by taking samples of the waveform randomly over time. The time between the trigger and each random sample is measured, and eventually enough random samples are obtained that the entire waveform can be reconstructed simply by displaying all of the samples with the phase relationship intact relative to the reference.
In the past, measurement of phase angle between the reference and the signal under analysis has used complicated and sensitive circuitry. One such circuit, used by Tektronix in its Model 2430, involves the use of two dual-slope integrators and counters. One integrator and counter is used for each half of the clock signal.
The difficulty with such circuits is that they are complicated and have critical adjustments. For example, see the description of the Tektronix circuit noted above as described at page 3-54 of the Service Manual for that unit. As a result, there has been a need for a simple, reliable circuit for detecting phase angle in the environment of digital oscilloscopes, and particularly digital storage oscilloscopes.